Messages in this thread | | | Date | Tue, 4 Mar 2008 16:26:26 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [patch] Re: using long instead of atomic_t when only set/read is required |
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On Wed, Mar 05, 2008 at 12:54:03AM +0100, Rafael J. Wysocki wrote: > On Wednesday, 5 of March 2008, Peter Hartley wrote: > > On Mon, 2008-03-03 at 18:24 +0100, Pavel Machek wrote: > > > Ok, I can understand the gcc side. But do we actually run on an > > > architecture where > > > > > > long *x; > > > > > > *x = 0; > > > > > > racing with > > > > > > *x = 0x12345678; > > > > > > can produce > > > > > > *x == 0x12340000; > > > > > > or something like that? I'm told RCU relies on architectures not doing > > > this, and I'd like to get this clarified. > > > > ARM6, ARM7500 and similar do exactly this for short (and unsigned > > short), although not for int, long, or pointers: > > > > > struct foo { short b; short c; }; > > > void baa(struct foo *f, short cc) { f->c = cc; } > > > > becomes (arm-linux-gcc -mcpu=arm6): > > > > > baa: > > > mov r3, r1, lsr #8 > > > strb r3, [r0, #3] > > > strb r1, [r0, #2] > > > mov pc, lr > > > > note the two single-byte stores, as ARM6 didn't have the "store > > halfword" instruction. > > > > So I think Alan Stern's > > "For all properly-aligned pointer and integral types other than long > > long..." > > should be amended to > > "For all properly-aligned pointer and integral types other than short or > > long long..." > > Well, perhaps it's sufficient to document just pointers? In fact this is what > RCU relies on.
One can do RCU on array indexes (ints/longs) as well as pointers, so we need to keep "integral" in there.
Thanx, Paul
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